New peppers will resist West's TSWV

Dan Bryant | Jun 04, 2005

A Hollister-based plant breeder says he expects next year to be testing new pepper varieties for resistance to tomato spotted wilt virus (TSWV), a disease occurring in half the pepper fields sampled in 2004 on California's Central Coast.

According to Bob Heisey of United Genetic Seed Co., his varieties will be tailored to the region. Commercial varieties having resistance to the thrips-vectored TSWV exist in the southeastern U.S. but are not adapted to California.

The resistance, however, is controlled by a single dominant gene and can be transferred, with some effort by breeders, to material suited for California pepper production.

Presently, the only defense against TSWV is discouraging various species of thrips, present throughout the growing season, by applying imidacloprid to young pepper plants. While the chemical does not control thrips directly, they are repelled by its taste. Treating for them on nearby harvested fields of radicchio, also a host of both the disease and vector, has been another approach.

TSWV symptoms include yellowed and stunted plants and fruit having chlorotic spots and yellow halos.

In a survey of six viruses in pepper fields in Santa Clara and San Benito counties in the fall of 2004, Aziz Baameur, Santa Clara County farm advisor, reported finding TSWV infections in 21 of the 42 fields sampled. Infection rates with various viruses were estimated at 5 to 75 percent.

Resistance work

Heisey told a recent gathering of pepper growers and PCAs at Gilroy he also is working on resistance to four other diseases of sweet and hot peppers: Phytophthora root and crown rot, powdery mildew, cucumber mosaic virus, and Verticillium wilt. He anticipates having material to test for these in two to five or more years.

Development of resistance requires three major elements, he explained. They are: (1) a source of resistance, often from wild plants, (2) inoculation of plants to be able to sort the resistant from the susceptible, and (3) adapted lines or varieties for use as parents in the breeding process to achieve acceptable horticultural types for commercial use.

“Managing pepper diseases by the use of genetic resistance, if such resistance is available,” he said, “is usually the simplest and most cost-effective method of disease control.”

However, he added, years of research and thousands of dollars must be spent in finding, characterizing, and introducing resistance in new varieties.

“It's a long, arduous process, and breeders, pathologists, and technicians work to bring resistances from unadapted, sometimes wild, relative species into types with the proper horticultural type for the California pepper industry.”

Turning to the other four diseases targeted for resistant varieties, Heisey said resistance to Phytophthora root rot is available in commercial peppers grown elsewhere but not accepted in California.

Expects to test

Incorporation of the resistance is complex and time-consuming, since both parents of a cross must carry the resistance, but he expects to be testing new varieties with high resistance within two years.

Heisey also anticipates in two years to be testing new materials able to fend off powdery mildew. The trait is thought to be carried by a single, dominant gene. Although multiple sources of resistance are available, the problem has been most sources are wild relative species not readily crossed with cultivated peppers.

However, a mildew-resistant, bell pepper-like source from a breeder outside the U.S. was successfully screened by Mike Coffey, plant pathologist at the University of California, Riverside, with support of the California Pepper Commission, and Heisey said it is being evaluated in the field this season.

Powdery mildew, a perennial problem for coastal pepper growers and target for commission research funds, is caused by a fungus, Leveilla taurica, and defoliates plants, exposing fruit to sunburn. Difficult to manage because it occurs with great variability, it has a strong ability to develop resistance to fungicides, none of which is an eradicant.

Lines from Asia and wild species were the original sources of resistance to CMV, a virus that comes and goes in California pepper fields. A persistent shortcoming of crosses thus far for this purpose is that not all plants of a population have resistance.

At least five more years of work, involving Cornell University and the pepper commission, will be needed to obtain CMV-resistant varieties having desired horticultural type. The resistance may be linked to small fruit size.

Vert resistance

About the same amount of time will be needed to come up with new varieties resistant to Verticillium wilt. How resistance to this disease is inherited is not clear to breeders, but breakthroughs to transfer it to bell and hot peppers were made a couple of years ago, and materials look promising as improvements to existing varieties.

Heisey said tobacco mosaic virus and bacterial spot, two minor diseases of peppers that occur sporadically on the Central Coast, also can be controlled by genetic resistance, as well as cultural practices.

The potyvirus complex of pepper mottle virus, potato virus Y and tobacco etch virus also has a history of striking coastal pepper fields from time to time. Resistance to pepper mottle, potato virus Y, and some races of tobacco etch are available.

Lest the pepper industry become complacent with resistant varieties once they are being used, Heisey warned that little is known about how pathogens adapt. A currently available resistance might not hold against local isolates of disease occurring in fields.

There is some evidence, he continued, that resistance to TSWV can be broken by an isolate, or strain, collected from fields in the Gilroy area. He said his company is using local isolates in its screening and re-isolates frequently from diseased plants.

Often by the time a new variety is developed and put into fields, a disease population makes an adaptation to challenge the resistance. The new isolate may then disappear or persist indefinitely.

And, he went on, growers can expect to pay more for improved varieties due to the intensive research in developing them.

Pay for protection

“Growers will have to figure those increases in their costs of production, but to avoid disasters like some fields we saw last year, they will certainly be willing to pay for a little insurance against diseases,” Heisey concluded.

Another speaker at the gathering, Ken Melban, director of crop protection for the pepper commission, said the body is continuing to pursue a full registration for Rally fungicide on peppers.

In the meantime, the industry has been using the material under a Section 18 registration, while triazole, a compound in Rally, is being evaluated.